In U.S. application Ser. No. 11/834,002, Sostaric et al. disclosed that plasticizers and air entraining agents can be dosed into the concrete contained in the mixing drum of a delivery truck using a process control system. However, it was not explained how rheological properties, on the one hand, and, air properties, on the other hand, could be simultaneously controlled.
It has been known, however, that “slump” or fluidity can be individually monitored and adjusted using a process control system. This is done by measuring the energy required for rotating concrete in a mixing drum using various sensors; correlating energy values with slump values (using a standard slump cone test); and storing this information in memory so that a computer processing unit (“CPU”) can correlate the energy and slump values. See e.g., U.S. Pat. Nos. 4,008,093, and 5,713,663. As concrete stiffens over time, due to hydration, evaporation, and/or other factors, greater energy is required to rotate the drum, and the CPU can be programmed for activation of devices to inject water or chemical dispersant into the concrete mix.
Numerous patents have declared that various properties of concrete mixes can be monitored and adjusted through the use of sensor devices that are connected to a CPU. For example, in U.S. Pat. No. 5,713,663, Zandberg et al. deployed sensors for measuring amounts of batch water and particulate ingredients, sand moisture content, time, and other factors (See e.g., col. 8, lines 3-14). In US Patent Publication No. 2009/0037026, Sostaric et al. referred to sensors for detecting mixing drum temperature, rotational speed, “acceleration/deceleration/tilt,” vibration, and other properties. These and other prior art publications contain the common suggestion that data gathered by the sensors can be listed in “look up” charts.
However, for all of these prescriptions concerning the usefulness of sensor-derived data, the monitoring and controlling of rheology (e.g., slump) and entrained air content have not been accurately or reliability achieved or integrated in practice. Although it is generally known that changing the air content affects slump and vice versa, the concrete industry has not able to predict what the slump of a concrete mix might be, for example, by doubling or multiplying entrained air levels. In other words, while slump might be increased as a general proposition by increasing air content, the precise extent to which slump is increased has not been reliably predicted based on how much air entraining admixture (“AEA”) is introduced into the concrete mix. As a result, prior art devices have focused only on adjusting slump or other rheological property in the truck.
For concrete without AEA, adding cement dispersant typically has negligible effect on air content and prior art devices were designed only for adjusting slump. However, in air entrained concrete, the present inventors realize that changing rheology by adding cement dispersants substantially affects air content. They believe that an integrated approach to controlling both rheology and air content is needed for such cases.
The problem is that there is no consistent or linear correlation between rheological properties such as slump and the use of AEAs in concrete mixes. This is due largely to the nature of concrete, which is inconsistent from batch to batch, and even from day to day. Many factors lead to this inconsistency: including variability in batch mix design, ingredient quality and source, processing conditions (e.g., temperature, humidity, revolutions needed for particular mixing drum), and nature of dispersant and AEA employed. As further explained hereinafter, chemical dispersants and AEAs may have adverse and unpredictable effects on the performance of the other.
The prior art is devoid of precise teachings about how to administer AEAs and dispersants using an integrated approach in a concrete mixer. The present inventors believe that the number of problems created by AEAs for ready-mix producers, contractors, and owners far exceed those created by all other admixtures. Almost everything influences the performance of AEAs: e.g., ambient and concrete temperatures, travel time from plant to site, mixing time, cement type, and the nature and variability of cement dispersants (particularly polycarboxylate superplasticizers).
There are different types of air in concrete: entrapped air and entrained air. “Entrapped air” results from the mixing process, whereby air is mechanically enfolded (usually 1.5% by concrete volume) by churning aggregates or moving paddles within the mixing drum. Such entrapped air is visible to the eye as irregularly shaped voids when viewed, for example, in a sawn cross-section of hardened concrete. On the other hand, “entrained air” has the form of microscopic, spherically shaped voids; it thus becomes more easily distributed throughout the mix. The sizing and spacing of entrained air bubbles is important for enhancing durability of concrete subjected to freeze/thaw conditions. Air entraining admixtures are used to form and to stabilize these microscopic voids in the concrete.
What complicates matters is that typical measurement of entrained air in concrete involves a percentage reading of the total amount of air, which is to say both “entrapped” and “entrained” air. Where the percentage reading is 6% air, for example, this means approximately 1.5% of the total air is entrapped, while 4.5% is entrained air.
Further complicating matters is the fact that the concrete mixing drum and the motion of the delivery truck, which is jostled by travel over irregular roadways and surfaces, can increase the relative amount of “entrapped” air. On the other hand, the longer the time that the concrete is being transported in the truck, the more the free water content in the concrete decreases due to evaporation and hydration. As water is necessary for air bubble formation, the percentage of total air, including entrained air, can decrease.
A still further complicating factor is that different kinds of AEAs have different effects on bubble formation in the concrete mix and can be influenced differently depending on the type of dispersant used.
A wood-derived salt type AEA, such as Vinsol resin, is often used in low water concrete mixes to obtain a good bubble structure. When a superplasticizer is added, such as to obtain a higher slump at pour, entrained air levels tend to decrease. This continues the longer the concrete is mixed in the truck. Addition of air entraining admixture is thus required for this situation.
On the other hand, a synthetic resin type AEA (e.g., fatty acid salts or tall oil) works differently in that it tends to entrain more small bubbles as slump increases. Thus, if water is added to the concrete mix at pour, the amount of entrained air can increase potentially to excessive levels in certain situations.
In view of the foregoing, the present inventors believe that a novel method and automated process control system is needed for integrating and simultaneously monitoring and adjusting both air and rheology properties, using separate component additions of an air control admixture component and a rheology control admixture component, which hitherto have had unpredictable and adverse effects upon each other and upon the resultant concrete mix.